Synthetic polymeric membranes are used for separation of species by dialysis, electrodialysis, ultrafiltration, cross flow filtration, reverse osmosis and other similar techniques. One such synthetic polymeric membrane is disclosed in Australian Patent Specification No. 505,494 of Unisearch Limited.
The membrane forming technique disclosed in the abovementioned Unisearch patent specification is broadly described as being the controlled uni-directional coagulation of the polymeric material from a solution which is coated onto a suitable inert surface. The first step in the process is the preparation of a "dope" by dissolution of a polymer. According to the specification, this is said to be achieved by using a solvent to cut the hydrogen bonds which link the molecular chains of the polymer together. After a period of maturation, the dope is then cast onto a glass plate and coagulated by immersion in a coagulation bath which is capable of diluting the solvent and annealing the depolymerised polymer which has been used. According to the one example given in this specification, the "dope" consisted of a polyamide dissolved in a solvent which comprised hydrochloric acid and ethanol.
In another membrane forming technique, the liquid material out of which the membrane is cast is a colloidal suspension which gives a surface pore density that is significantly increased over the surface pore density of prior membranes.
According to that technique a thermoplastic material having both relatively non-crystalline and relatively crystalline portions is dissolved in a suitable solvent under conditions of temperature and time which cause the relatively non-crystalline portions of the thermoplastic material to dissolve whilst at least a portion of the relatively crystalline portion does not dissolve but forms a colloidal dispersion in the solvent. The colloidal dispersion and solvent (i.e. the "dope") is then coated onto a surface as a film and thereafter precipitation of the dissolved thermoplastic portion is effected to form a porous membrane.
Membranes of both of the above kinds suffer from disadvantages which limit their commercial usefulness and applicability. For example, they exhibit dimensional instability when drying and may shrink by up to 7%. Thus, it is essential that they be kept moist prior to and after use. Furthermore, where the membranes are made from polyamide, it has not been possible to generate concentrated and varied chemical derivatives of the membranes and this restricts the situations to which the membrane may be applied.
Another disadvantage is that such polyamide membranes are fundamentally unstable and eventually become brittle on storage. The instability has been carefully investigated by I. R. Susantor of the Faculty of Science, Universitas Andalas, Padang, Indonesia with his colleague Bjulia. Their investigations were reported at the "Second A.S.E.A.N. Food Waste Project Conference", Bangkok, Thailand (1982) and included the following comments regarding brittleness:
"To anneal a membrane, the thus prepared membrane (according to Australian Pat. No. 505,494 using Nylon 6 yarn) is immersed in water at a given temperature, known as the annealing temperature, T in degrees Kelvin. It is allowed to stay in the water a certain length of time, calling the annealing time. For a given annealing temperature, there is a maximum annealing time, t(b) in minutes, beyond which further annealing makes the membrane brittle. Plotting 1n 1/t(b) versus 1/T gives a straight line. From the slope of this line it can be concluded that becoming brittle on prolonged annealing is a process requiring an activation energy of approximatey 10.4 kilocalories/mole. From the magnitude of this activation energy, which is of the order of van der Waals forces, the various polymer fragments are probably held together by rather strong van der Waals forces or hydrogen bond(s)." PA0 (i) dissolving an aliphatic polyamide or an aliphatic polyamide/polyimide copolymer which has both relatively non-crystalline and relatively crystalline portions into a solvent under conditions of temperature and time which cause the relatively non-crystalline portions of the polyamide or copolymer to dissolve while at least a part of the relatively crystalline portions of the polyamide or copolymer do not dissolve, but form a colloidal dispersion in said solvent, PA0 (ii) forming said colloidal dispersion and solvent into a film and thereafter causing precipitation of at least part of the dissolved non-crystalline portions in the film to form a porous membrane in which the pores are defined by spaces between the relatively crystalline portions, and, PA0 (iii) reacting the membrane with an aldehyde as herein defined to link at least some of the relatively crystalline portions with the aldehyde. PA0 (a) reacting at least some of the remaining reactive single-link aldehyde chains with hydrazine, PA0 (b) reacting the phenolic hydroxyl groups with epichlorohydrin, PA0 (c) reacting the resultant epoxides with a diamine to fix a pre-determined concentration of amine groups hydrolysing excess epoxide groups to hydroxyls and, PA0 (d) reacting the amine groups with excess bis(isothiocyanate).
We have confirmed that the brittleness is due to a recrystallization of water-solvated amorphous polyamide. In some cases (such as polyamide 6) brittleness occurs within 48 hours of immersion in distilled water (pH 7) at 80.degree. C. Colorimetric --NH.sub.2 end group analysis has shown that there is no significant hydrolysis of the amide groups during this time. As would be expected, the rate of embrittlement is catalysed by dilute acids (eg: pH of 1.0) due to nitrogen protonation and subsequent solvation. This effect explains the apparently low "acid resistance" of the polyamide membranes. However colorimetric determination of both --NH.sub.2 end groups and --COOH end groups has shown that the effect is due to crystallization rather than acid catalysed hydrolysis.
There is a potential source of confusion in the use of words such as "acid-resistance" in the context of this specification. That most of the brittleness is due to physical effects rather than chemical decomposition or chemical solvation (at least for dilute acids) is shown by the extreme embrittlement caused on standing 5 minutes in absolute ethanol. The ethanol removes the plasticizing water tenaciously held by non-crystalline nylon as will hereinafter be described in relation to Example 2. Accordingly, the following definitions apply in this specification:
(a) "Embrittlement resistance" means hindrance or prevention of the physical recrystallization mechanism of the amorphous polymer matrix.
(b) "Acid-catalysed embrittlement resistance" means prevention of embrittlement of type (a) even in the presence of dilute acids (pH 1 to 7).
(c) "Acid solubility" means the rapid dissolution of polyamide in strong acids (100% formic acid or 6N hydrochloric acid).
(d) "Acid catalysed hydrolysis" means the scission of amide bonds (such scission is much faster in an amorphous polyamide than in a crystalline polyamide.)
As well as "embrittlement" the prior art membranes show the normal chemical defects of the parent nylon polyamides in that they possess only moderate oxidation resistance and bio-resistance.
It is an object of this invention to provide polymeric porous membranes composed of thermoplastic aliphatic polyamides (including polyamide/polyimide copolymers) which have greater resistance properties and improved mechanical stability than prior art membranes. It is a further object of the invention to provide polymeric porous membranes which readily lend themselves to the preparation of chemical derivatives thereof for particular uses.